The invention relates generally to sampling an electrical signal, and more particularly to short duration sampling of an input electrical signal and short duration pulse generation.
The generation of pulses and the generation of sampling apertures is important in a number of systems. With the operating frequency of systems increasing due to technological improvements, it becomes important to provide narrower or xe2x80x9cultra-fastxe2x80x9d pulses and sampling apertures to match such increased operating frequencies of systems. A few exemplary systems where such xe2x80x9cultra-fastxe2x80x9d pulses or sampling apertures are desirable include fiber optic communication systems, radar communication systems, and time domain reflectometers.
Prior art systems for generating fast pulses have relied on expensive means such as step recovery diodes (SRDs), which generate a fast rising electrical voltage step that is then converted into an impulse by one or more capacitors, which differentiate the step, or a pulse-reversing device such as a shorted transmission line, which provides a delayed, inverted edge which cancels out the initial edge after a brief delay. The resulting pulse can be used to generate a sampling aperture by the use of a Schottky diode bridge whose diodes are turned on momentarily by the energy in the pulse. Sampling oscilloscopes such as the Tektronix 11801/CSA803 or the Agilent 54750A use such techniques. Another prior art technique for generating fast sampling apertures is to use a Gilbert Multiplier circuit which multiplies an input waveform by a fast pulse, such as one generated by a step recovery diode and a shorted transmission line or capacitor.
The above prior art techniques such as the SRD-based circuits suffer from variations in pulse position with temperature, and moderately high amounts of jitter. Secondly, the variable input impedance of the Schottky diodes is sub-optimal for input impedance matching. Additionally, the SRD requires several nanoseconds of carrier charging and discharging time, limiting the pulse-repetition rate to approximately 100 MHz. Also, it is difficult to incorporate into monolithic designs due to the constraints imposed in fabricating the step recovery diode, capacitors, and Schottky diodes on the same substrate. Finally, these circuits can be damaged easily by application of an over-voltage to the input.